0099-2399/89/1604-0158/$02.00/0 JOURNAL OF ENDODONTICS Copyright 9 1990 by The American Association of Endodontists

Printed in U.5 'L VOL. 16, NO. 4, APRIL 1990

Root Canal System of the Maxillary Central Incisor Etsuo Kasahara, DDS, DDSc, Eiichi Yasuda, DDS, DDSc, Akio Yamamoto, DDS, and Masaaki Anzai, DDS

Another area in need of further research concerns the condition of accessory canals such as lateral branches. Whereas the high incidence of these branches has been noted, especially for the apical portion of the root (l 1-13), their size as a possible factor in endodontic failure has not been investigated. Data on the incidence of apical ramifications are even more sparse. This article reports the results of a study of extracted maxillary central incisors, prepared with the dye-and-cleating method, and examined with the naked eye for the thickness and curvature of the root canal, the condition of any accessory canals, and the location of the apical foramen. The purpose of this research was to provide clinical data on root canal anatomy that will enable more accurate assessment of the efficacy of mechanical instrumentation and thereby provide better guidelines for preparation of the root canal in endodontic treatment.

To better assess the efficacy of mechanical preparation of root canals, transparent specimens of 510 extracted maxillary central incisors were investigated for thickness and curvature of the root canal, condition of any accessory canals, and location of the apical foramen. Over 60% of the specimens showed accessory canals that are impossible to clean mechanically. Most lateral branches were small, 80% were the size of a # 1 0 reamer or less, and only 3% were thicker than a # 4 0 reamer. Apical foramina located away from the apex were observed in 45% of the teeth, although nearly 80% of all foramina were within - 0 . 5 mm of the apex, and 95% were within - 1 . 0 mm. Data on the thickness and curvature of the main canal showed that normally it is adequately prepared when reached with a # 6 0 reamer to the apical constriction and supplemented by flare preparation.

M A T E R I A L S AND M E T H O D S This study used 510 extracted maxillary central incisors, each with no visible abnormalities. The teeth were subjected to manual surface cleaning, then placed in a 10% sodium hypochlorite solution in an ultrasonic cleaner to dissolve away remaining organic surface debris. After cleaning, a radiograph of each tooth was taken from the labio-lingual direction, and a dye (India ink) was injected by the vacuum injection method (I 3). The teeth were decalcified with 10% nitric acid for 48 h, then washed and dried. Transparent specimens were made by preserving the teeth in methylsalicylate. The following assessments of these specimens were then made with the naked eye.

One of the most important factors in endodontic therapy is the mechanical preparation of the root canal itself. Although it has been reported that high success rates are obtained by adequate endodontic procedures (1, 2), no suitable method has been demonstrated for cleaning complex root canals such as those having lateral branches and apical ramifications (3), and the cleaning of narrow flattened canals or roots with intense curvature is extremely ineffective (4-6). Although the anatomy of root canals has been well documented (7), research on the implications of this anatomy for adequate endodontic treatment remains insufficient. Accurate data on the size of the apical portion of canals, for example, would permit reliable criteria for the amount of instrumentation normally required in this region. To provide such data, Kerekes and Tronstad (8-10) measured the diameters of root canals on sections taken at 1, 2, 3, 4, and 5 m m from the apexes of anterior, premolar, and molar teeth. They then estimated the smallest sizes of instruments that would assure, with 90% probability for each type of tooth, adequate preparation o f circular shaped canals at various distances from the apex. The reliability of these findings is compromised by the small size of their sample; for each type of tooth they examined only 20 specimens. They moreover based all measurements on the distance from the anatomical apex, rather than from the actual foramen, and accordingly their data are not uniform with regard to the anatomy of the root canal per se.

Accessory Canals The number of accessory canals (apical ramifications and lateral branches) was noted for each specimen. Each lateral branch was classed according to the level of its opening on the root surface, which was divided for this purpose into sixths, consecutively numbered from the apical (1/6) to the cervical (6/6) portions. The orientation of each opening was classified according to criteria set forth by Yoshiuchi et al. (13). On a hypothetical cross-section of a root having a lateral branch, the middle of the labial root surface is designated as 0 o'clock, the center of the lingual surface as 6 o'clock, and so forth. The labial (buccal) surface (B) is then defined as the span between 11 and 1 o'clock, the mesiolabial (BM, or distolabial, BD) as 1 to 2 o'clock, the mesial (M, or distal, D) as 2 to 4 o'clock, and so on, and the orientation of the lateral 158

Vol. 16, No. 4, April 1990

Canal Anatomy

branch is classified according to these divisions (Fig. 1). Lateral branch thickness was classed into five sizes, according 1o whether it appeared to the eye as smaller than a #10 reamer, roughly equal to a #10, roughly equal to a #15, equal to or greater than a #20, up to a #40, or larger than a #40 (Fig. 2).

Apical Foramen The straight line distance of the foramen from the apex was measured and classed according to 0.5-mm intervals. Teeth in which the apical foramen was eccentrically positioned were examined for the orientation of the orifice, using the same criteria described for lateral branch orifices. In cases of apical ramification, when a branch could be judged as the one that a probe would most likely follow, its orifice was defined as the apical foramen, and its distance from the apex and orientation were treated in the above manner.

Main Root Canal The curvature of the main root canal was measured as the angle between the axis of the canal (as it extends away from

B. B.D. ~B . M . )

.M.~ B.D.

M.~ D.)

D. ~M,)

159

the pulp cavity) and a line connecting the foramen with the point where the canal may be discerned to diverge away from the axis. The curvature was classed in 10-degree increments (0 to 9, 10 to 19, 20 to 29, 30 or greater). The direction of curvature was classified as toward either the labial, lingual, mesial, or distal surface. The thickness of the main root canal was measured as its widest diameter, regardless of the direction of measurement, at intervals of 1 m m over the first 5 m m from the apical foramen. The measurements were divided into less than a #20 reamer, from a #20 to less than a #40, from a #40 to less than a #60, from a #60 to less than a #80, and more than a #80. RESULTS

Accessory Canals Of the 503 teeth for which evaluation was possible, well under half had simply a single main canal (Table 1). Apical ramifications were observed in 62 teeth; of these 51 had only two branches, 6 had three and 5 had four or more (Fig. 3). More c o m m o n was the presence of lateral branches, the majority of which were located in the -'/6 or 3/6 sections and opened on the labial, mesiolabial, or mesial surfaces (Table 2). Of all teeth with lateral branches, about half (47.3%) had only one, a quarter (25.1%) bad two, and smaller incidences had three (13.1%) or four or more (14.5%). Data on the thickness of the lateral branches are summarized in Table 3. No relation was found between lateral branch thickness and the orientation of the orifice. Because the incidence of lateral branches decreased in inverse proportion to their thickness, the number of branches TAeLE 1. Classification of root canal system

L

.~L.D.)

L. FIG 1. Orientation of lateral branch orifice.

Type Simple main root canal Apical ramification only Lateral branch(es) only Both apical ramification and lateral branch(es) Total

F~G 2. Teeth having lateral branches. Left, cleared specimen showing three lateral branches in the 1/8 portion. Thickness of branches, in ordor going toward apex, was classed as roughly equal to a #10 reamer, smaller than a #10, and roughly equal to a #15. Middle, tooth with a branch in the 2/8 portion, whose thickness was classed as equal to or greater than a #20, up to a #40 reamer. Right, tooth with an extremely large branch, larger than a #40 reamer, in the 3/6 portion.

No.

%

194 34 247 28

38.6 6.7 49.1 5.6

503

FiG 3. Teeth having apical ramifications. Left, cleared specimen showing two canals. Right, tooth with four canals.

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Kasahara et al.

Journal of Endodontics

TABLE 2. Orientation and level of lateral branch orifice

TABLE 4. Orientation of eccentrically located apical foramen

Orientation

Orientation

Level

Total B

BM

BD

4/6 3/6 2/6 1/6

12 70 117 44

4 33 58 12

0 6 19 5

L

Total %

243 107 30 21 23 9 125 18 576 42.2 1 8 . 6 5.2 3.6 4.0 1.6 21.7 3.1

0 0 11 10

LM

LD

M

D

0 6 12 5

1 1 6 1

11 47 46 21

3 3 11 1

%

31 5.4 166 28.8 280 48.6 99 17.2

No. %

B

BM

BD

L

LM

LD

M

D

58 12.4

28 6.0

18 3.9

19 4.0

12 2.5

13 2,8

42 9.0

20 4.3

TABLE 5. Degree and direction o f c u ~ a t u r e of main rootcanal Direction Degree

TABLE 3. Thickness of lateral branch Thickness

No.

%

Less than #10 reamer Approximately equal to # 10 Approximately equal to #15 From #20 to #40 Larger than #40 reamer

325 142 56 38 15

56.4 24.7 9.7 6.6 2.6

Total

B

L

M

>30 20-29 10-19 0-9

2 9 19

0 5 28

0 0 1

Total %

30 41.7

33 45.8

1 1.4

Total

%

0 0 8

2 14 56 438

0.4 2.7 11.0 85.9

8 11.1

510

D

576

whose thickness exceeded a #40 reamer was small. All 15 of these largest branches had orifices in the 2/, and 3/0 portions of the root. Only nine were visible radiographically, however. The six that escaped detection on the radiograph were located on or near the labio-lingual axis of the teeth. Four of these were located on the labial surface; a single branch was found on the lingual surface, and one was on the mesio-labial surface.

Apical Foramen Of the 468 teeth with a distinct apical foramen, 258 (55.1%) had the foramen located at the center of the root apex. Of the remainder, the foramen was judged to be approximately 0.5 mm away from the apex in 115 teeth (24.6%), at approximately I m m in 71 (15.2%), at 1.5 m m in 16 (3.4%), and 2 mm or more in 8 (1.7%). Thus, a total of 79.7% of all foramina were located approximately 0.5 m m or less from the apex and 94.9% were approximately 1.0 m m or less away. Data on the orientation of the eccentrically located foramina are given in Table 4. As was true for the orifices of lateral branches, most were found to range from the mesial to the labial surfaces.

Main Root Canal Data on the degree and direction of curvature of the main root canal are given in Table 5. The vast majority were straight or nearly so (

Root canal system of the maxillary central incisor.

To better assess the efficacy of mechanical preparation of root canals, transparent specimens of 510 extracted maxillary central incisors were investi...
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